Such rostra typically include a latching mechanism for locking frame units with respect to one another in the extended position. A latching mechanism is described in WO 2011/042709 which comprises a movable blade which is engageable with a latch surface. Each frame unit has a respective movable blade and latch surface and is configured so that, during retraction, the smallest frame unit is first of all unlatched and moved into the next largest frame unit, unlatching it in the process. As a result, the frame units are unlatched and retracted one by one from the smallest to the second largest.
The present inventor has realised that this system, though very widely used, has a practical problem. In particular, in order to ensure that the frame units are snugly and safely received within one another, they have to be guided carefully during retraction. There can be a tendency to misalign slightly. During retraction, operators have to be careful to guide the rostra and correct any misalignments.
The present inventors have realised that a latching mechanism as described in WO 2011/042709 has the disadvantage that, as retraction proceeds, the number of frame units which require guiding increases. This means that the total mass which requires guiding increases, so that, at the end of the retraction process it can be very difficult and requires a lot of effort to ensure that the frame units are stacked smoothly one inside another.
The present inventors have set out to overcome this problem.